Method and apparatus for influencing the temperature of a fluid leaving a heat exchanger



March 9, 1965 A. BRUNNER 3,172,462

METHOD AND APPARATUS FOR INFLUENCING THE TEMPERATURE OF A FLUID LEAVING A HEAT EXCHANGER Filed Nov. 16, 1960 4 Sheets-Sheet 1 v ii SET Pam/r J/GNAL ---l 7 A a 4* d A F [:9- 3 p I I I 32 I 3 33 W 36 5 53 .7nvenf'0r: 14L FRED BPUNNEE.

A. BRUNNER March 9, 1965 METHOD AND APPARATUS FOR INFLUENCING THE TEMPERATURE OF A FLUID LEAVING A HEAT EXCHANGER 4 Sheets-Sheet 3 Filed Nov. 16, 1960 March 9, 1965 A. BRUNNER 3,172,462

METHOD AND APPARATUS FOR INFLUENCING THE TEMPERATURE OF A FLUID LEAVING A HEAT EXCHANGER Filed Nov. 16, 1960 4 Sheets-Sheet 4 27g x EL /253 6'6 Q 56 R259 .957 Pol/W 51 0mm K Jn ven fo r:

A 1. F'IPED BPUNNEIP.

United States Patent tion of Switzerland Filed Nov. 16, 1960, Ser. No. 69545 Claims priority, application Switzerland, Nov. 20, 1959,

,8s4 21 Ciaims. (Cl. 165-1) The present invention relates to a method and apparatus for influencing the temperature of a fluid leaving a heat exchanger wherein heat is exchanged between said fluid and a second fluid. The method and apparatus according to the invention are particularly suitable for influencing the temperature of steam leaving a superheater of a steam generator.

Several systems acting either on the primary side or on the secondary side of a heat exchanger are known tor influencing the temperature of one of the heat exchanging fluids leaving a heat exchanger. It is conventional, for example, to inject a cooling or heating fluid into one of the heat exchanging fluids entering a heat exchanger for influencing the temperature of this fluid at the outlet of the heat exchanger. It is also known to conduct a measured portion of a fluid through a conduit by-passing all or part of a heat exchanger so that the by-passed portion of the fluid does not take part at all in the heat exchange, or participates only partly in the heat exchange. It is also possible to suplement'ally cool or heat a heat exchanging fluid according to the desired final temperature of the fluid before the fluid enters the main heat exchanger. It is possible to change the active heat exchange surface or the amount or temperature of the heating or cooling agent for the heat exchanger in order to obtain the desired final temperature of the fluid which is heated or cooled in the heat exchanger. Deviations of the temperature of a fluid leaving a heat exchanger from a desired temperature or set point temperature are usually taken care of :by a control device which is responsive to the deviation. Injecting a coolant or a heating medium into the inlet conduit of a heat exchanger has the advan tage of influencing the temperature of the fluid entering the heat exchanger without susbtantial delay.

Heat exchangers, particularly heat exchangers forming part of larger steam generators, usually consist of a plurality of pipe lines arranged in parallel with respect to the flow of a heat exchanging fluid therethrough. These pipe lines must be united into a single pipe to aflord injection of a coolant into the fluid and the fluid must be distribute/.1 into a plurality of pipe lines after the injection, because it is usually much too complicated and too expensive to provide injection into each of a plurality of pipe lines. Coolant injection is, therefore, usually done between existing collectors and distributing headers. In many cases the last intermediate collector in a superheater is so far from the outlet of the superheater that the regulated superheater section is large and constitutes an undesired inertia for the control operation.

In order to overcome this last mentioned disadvantage, it has been proposed to produce an advance signal at an intermediate point of a heat exchanger which signal corresponds to an intermediate temperature of a heat exchanging fluid, and to influence the temperature of the fluid leaving the heat exchanger in response to this signal and to a signal corersponding to the outlet temperature of the heat exchanger. The disadvantage of this system is that the control operation initiated by an undesired change of the intermediate temperature weakens the signal corresponding to the intermediate temperature and v ICC impairs its effect; this works against a further reduction of the deviation of the actual temperature from the desired temperature caused by the inertia of the system.

It is an object of the present invention to provide a method and apparatus for influencing the temperature of a fluid leaving a heat exchanger wherein heat is exchanged between said fluid and a second fluid which method and apparatus avoid the aforedescribed disadvantages of conventional systems. In the system according to the invention the temperature of one of the heat exchanging fluids leaving the heat exchanger is influenced in response to the difference between the temperatures of the fluid at two spaced locations of the passage of the fluid through the heat exchanger. If desired, temperature differences at various portions of the passage of the fluid through the heat exchanger may be used for influencing the temperature of the fluid leaving the heat exchanger.

The method according to the invention provides a very quick-acting correction of a temperature disturbance whereby the temperature influencing operation according to a temperature difference as described in the paragraph next above and according to one or more additional operating characteristics of the heat exchanger, such as the temperature of the heated fluid leaving the heat exchanger is very little or not at all affected by the change of the inlet temperature effected by the control operations according to the invention.

The apparatus according to the invention includes temperature sensitive devices for measuring the mean temperature of a fluid at two spaced locations of the passage of the fluid through a heat exchanger at which locations the fluid has different temperatures, a comparison device producing a signal corresponding to the difference of the measured temperatures, means producing a signal corresponding to the outlet temperature of said fluid, and means which influence the temperature of the fluid leaving the heat exchanger and operatively connected to the comparison device and to said means for producing a signal corresponding to the outlet temperature of the fluid for actuating said temperature influencing means according to the signal produced by said comparison device and to the signal corresponding to the outlet temperature of the fluid.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood trom the following description of embodiments thereof when read in connection with the accompanying drawing, wherein:

FIG. 1 is a diagrammatic illustration of a system according to the invention.

FIG. 2 is a diagrammatic illustration of a modified system according to the invention.

FIG. 3 is a diagram illustrating the temperature conditions in the system shown in FiG. 2.

FIG. 4 is a diagrammatic illustration of another modification of the system according to the invention.

FIG. 5 is a diagrammatic illustration of a further modification of the system according to the invention.

FIG. 6 is a diagrammatic illustration of yet another modification of the system according to the invention and more clearly showing structural features of control apparatus which may be used in the other illustrated modifications of the system according to the invention.

FIG. 7 is a diagrammatic illustration of another modification or" the system according to the invention.

Referirng more particularly to FIG. 1 of the drawing, numeral 1 designates a tubular heat exchange surface, for example, a superheater of a steam generator. A fluid to be heated is supplied to the tubular surface of an inlet conduit 2 and the heated fluid is received frdin the tubular heat exchange surface by an outlet pipe 3. A second or heating fluid, for example a hot gas produced by the combustion of fuel, is passed over the heat extemperature sensitive device 9 which responds to the tem perature of the first fluid leaving the heat exchanger through the outlet pipe 3.

Regulators and signal transfer devices having a proportional-integral characteristic are marked PI and regulators and signal transfer devices having a proportional characteristic are labeled P in all figures of the drawing.

Temperature sensing devices 12 and 11 measure the temperatures of the first fluid at two consecutive, spaced.

locations 19 and 20 of the passage of the first fluid through the heat exchanger 1 and transmit signals corresponding to the temperatures through conduits 14 and 15 to a comparison device 13 which produces a signal corresponding to the difference of the temperatures at the locations 1? and 20. The last mentioned signal is conducted through a conduit'lfi to a proportional regulator 17 which produces a signal which is conducted 'through a conduit 18 connected to the conduit 7 so that the signal produced by the regulator 8 is superimposed 'on the signal produced by the regulator 17. e i

In all figures of the drawing the and signs at points where two or more signal conduits are connected indicate the sign of the respective signal in the algebraic sum of the signals which sum is represented by a new signal.

It the heat transferred to the heat exchanger surface 1 increases, for example, because the temperature of the combustion gas increases, the difference between the temperatures at the locations 19 and 20' increases with relatively little delay. Therefore, a correcting signal is quickly transmitted to the motor operator of the injection valve 6, causing an increase of the amount of injected fluid. The resulting reduction of the temperature of the fluid passing through the inlet conduit 2 has little influence on the difference of the temperatures at the locations 19 and 20 so that this temperature difference is not reduced. Therefore, the signal produced by the difference of the temperatures at the locations 19 and 20 has the character of an advance signal which assists, in an anticipatory manner, the control of the variable which must be regulated.

In FIG. 2 numeral 31 design-ates a heat exchanger having an inlet conduit 32' and an outlet pipe 33, a conduit 34 provided with a valve 35 being connected to the inlet conduit 32 for injecting cooling water thereinto. It is assumed that the heat exchanger is the super-heater of a steam generator. In this.- case, the coolant supplied through the conduit 34 may be taken from the feedwater of the generator. A quick-acting temperature control circuit is provided for actuating the valve 35. The circuit includes a device 3& sensing the temperature at the point 37 of the inlet conduit 32. The signal produced by the device 3-5 is conducted by a conduit 38 to a proportional-integral regulator 39 wherefrom a signal is conducted through a conduit 49 to the motor operator of the injection valve 35.

The temperature of the steam leaving the heat exchanger 31 through the pipe 33 is influenced according control valve 35, is adjusted accordingto saidtempera ture' difference. To accomplish this the conduit 381' for 1 the signal corresponding to the tempenatur'e at the point 1 3'7 i's'connec-ted through a conduit 4-1 toa signal compari-- son device" 42' wherein the signal corresponding tothe temperature at 37 is subtracted from a signal conducted 1 to the device 42 through a conduit 43..and produced in a '1 device '45 which senses the temperature at an intermediate point 44- of the heat, exchanger. The comparison device to the difference between the temperatures at two spaced locations in the path of the operating medium flowing through the heat exchanger whereby the set point for the 42 produces a signal corresponding to the difference =be- A tween the temperatures of the operating medium at the? locations 37 and 44 the last mentioned signal being manducted through aco-nduit 46 to a multiplication device 47 .The signal produced 'in the device 4215'- multiplied in.

: the'device47 by asi'gn al 'co nducted to the latter through:-

a conduit 48 and? the produced signal is conducted i through a conduit 49rd a devicefiij wherein theset point for the temperature at37 is formed}- In'the device 5% the signal arriving thrcugh condiiit-AQis' subtracted from a; signal arrivingthrough a conduit 51 and representing the set point of theriesired temperature of the steam at the,

point 52 in the outlet pipe/-13. v

The multip-licationfaptor signal receivedin the multiplication device 47 through a conduit 48- is produced in: a device 58 which cornparesh signal corresponding to; the actual steam temperature at point SZ-and producedi in a temperature sensing device 53 and conducted from: the latter through conduit 54 into the device :58- with a: signal corresponding to the set point or desired tempera-.- ture at 52 and arriving through conduits 51 and When there is equilibrium, i.e., when there is no devia-- tion of the actual temperature vat 52 from the desiredi temperature, the signal in'the conduit 48 is constant and the multiplication factor in the device 47 is not changed... in this case the factor is suc'h that the signal produced in the device 47 corresponds to the difference between the temperatures of the medium entering the heat exchanger: and leaving the heat exchanger and extrapolated from; the difference between the temperatures at the locations:

. 37 and 44. The device 58 preferably has a proportional-.

integral character. 7

The diagram-shown in FIG. 3 is illustrative of the; operation of the system-shown in FIG. 2. The abscissa: of the diagram represents the path of the fluid to be; heated through: the heat exchanger; The ordinate A, represents the temperature of the fluid at the inlet of' the heat exchanger. The line .a shows the temperature increase of the fluid while it passes through the heat; exchanger, the ordinate B representing the temperature: of the steam at the outlet of the heat exchanger. The temperature A is equal to the desired temperature at the: location 37 and the temperature B is equal to the desired. temperature at the location 52. The difference .of the temperatures of the medium between thelocations 37 and." 44 is represented by the vertical At The signal in the conduit 49 corresponds to the temperature difference: B-A which is extrapolated from the value At The value of the signal in the conduit 49 is subtracted from. the signal arriving in the device 56 through the conduit 51'. and corresponding to the set point temperature .B at 52.. As long as the actual temperature B is equal to the extrapolated temperature Bthe set point of the regulator 39 remains adjusted for the temperature A and the opening of the injection valve 35 is not changed.

It is assumed that due to a disturbance the temperatur difference At increases to a value At so that the temperatures of the fluid passing through the heat exchanger corresponds to the dotted line b, and the extrapolated outlet temperature increases to'C; This undesired high temperature, however, does not occur at the location 52' ecause, according to the invention, the temperature difference CA extrapolated in the multiplication device t? on the basis of A5 is subtracted from the setpoint' temperature B in the device and a value D for the temperature of the medium entering the heat exchanger is obtained. This value represents the new set point of the regulator 39 and causes opening of the injection valve whereby the temperature at the location 37 is quickly dropped to the new temperature D. Thereupon the temperatures of the fluid passing through the heat exchanger correspond to the line d.

The heat transfer behavior of a heat exchanger depends on several different factors, such as size and length of the heat transfer surface, diameter and wall thickness of the tubes forming the heat transfer surface, rate of flow, i.e. flow velocity of the fluid to be heated or cooled, characteristics of the heat exchanging fluids and of the material of which the heat exchanger is made, as well as the heat transfer between the fluids and the heat transferring wall therebetween.

It is advisable to consider the heat transfer behavior of the heat exchanger when forming the signal corresponding to the difference between the temperatures of the fluid at intermediate locations of the passage of the fluid. This is done by influencing the signal produced by the lower temperature before this signal enters the comparison device 42. In the embodiment of the invention shown in FIG. 2 a delaying device 57 is interposed in the conduit 41. The delay produced by this device may correspond to the time required by a fluid particle for its travel from the location 37 to the location 44. In this way the effect of the correction, i.e., the change of the temperature of the fluid at the location 37 by injection of a coolant on the comparison device 42 is delayed until the effect of the correction is also felt at the location 44. This arrangement does not take care of other factors influencing the heat transfer behavior of the heat exchanger, for example the heat accumulated in the heat exchanging walls. It is, therefore, desirable that the device 57 does not only provide a compensation for the time it takes the fluid to travel from 37 to 44, but also for the entire heat transfer behavior of the heat exchanger between these locations, as will be described in connection with FIG. 4.

FIG. 4 diagrammatically illustrates an embodiment of the invention including a heat exchanger having a plurality of tubular heat transfer elements 61a, 61b, 61c, 61e, 61 which are arranged in parallel relation with respect to the flow 'of one of the heat exchanging fluids therethrough. The fluid is supplied to a header 63 through an inlet conduit 62 and is distributed by the header into the tubes 61a to 61 The fluid leaving the tubes is collected in a collector 64 and conducted therefrom into an outlet pipe 65. The parts of the heat exchanger located Within the zones 66 and 67 indicated by shade lines do not take part in the heat exchange and may be insulated against the heating medium indicated by arrows 78. The system shown in FIG. 4 includes a quick control circuit similar to that provided in the modification shown in FIG. 2 for controlling the inlet temperature of the fluid. This quick control circuit includes a temperature sensitive device 69 which is responsive to the temperature at the location 68 in the inlet conduit 62. The signal produced by the device 69 is conducted to a regulator 71 through a conduit 70, the regulator producing a signal which is conducted through a conduit 72 to the motor operator of a coolant injection valve 73 which is interposed in a pipe 74 for injecting coolant into the inlet conduit 62..

In contradistinction to the arrangement shown in FIG. 2, the part of the heat exchanger where the difference between the temperatures of the heat receiving fluid at two spaced locations of the passage of the fluid through the heat exchanger is measured is not at the beginning of the passage but at the end thereof. The dash-dot line 75 in FIG. 4 indicates a plane traversing the tubes 61a to 61 at locations where the lower temperature is measured and numeral 76 indicates the location where the higher temperature is measured. The temperature of the fluid traversing the plane 75 is measured by insulating 6 the tube 61 between the location 77 where the tube 61f intersects the plane '75 and the end of the tube 61) against heating by the fluid 7 8 and by measuring the tem perature at the location 79, i.e. at the end of the tube line 61f where, due to the insulation, the temperature of the fluid is substantially the same as at the point 77. g

The portion of the tube 61f betwen the points 77 and 79 has a similar effect as the delaying device 57 in FIG. 2. Since the path of the fluid from the location 79 to the location 76 can be neglected with respect to the heat transfer behavior of the heat exchanger, a disturbance of the fluid entering the heat exchanger, for example caused by adjustment of the temperature set point of the regulator 71, occurs almost simultaneously and with the same intensity at the locations 79 and 76. The temperature disturbance, therefore, has no substantial influence on the temperature signal which corresponds to the difference of the temperatures of the fluid at the locations 79 and 76.

In order to indicate the difference between the temperatures of the fluid at the locations 76 and 79, two temperature sensitive devices 80 and 81 are provided. The signal produced by the device 81 is conducted to a comparison device 82 through a conduit 83 and is subtracted in the comparison device from the signal produced by the device 80 and conducted to the device 82 through a conduit 84. The signal produced by the device 82 and corresponding to the result of the aforesaid subtraction is conducted through a conduit 85 to a multiplication device 86. The signal corresponding to the result of the multiplication is conducted through a conduit 87 and a conduit 88 to the regulator 71 for adjusting the temperature set point of the latter, i.e., for adjusting the temperature at the location 68 to be produced by the operation of the injection valve 73. The signals produced by the device 89 are also conducted through a conduit 89 to a regulating device 90 having a proportional and integral character. The device 9% produces a signal corresponding to the deviation of the temperature at the location 76 from a set point temperature supplied to the device 90 through a conduit 92. The signal produced in the device 90 is conducted by a conduit 91 which terminates in the conduit 88 so that the signal produced in the device 90 is superimposed on the signal produced by the device 86. An increase of the temperature difference of the fluid between the locations 77 and 76 and an increase of the temperature at the location 76 above a desired temperature reduces the set point of the regulator 71 and a decrease of the temperature dilference as well as a decrease of the temperature at the location 76 below the desired temperature at the location 76 increases the set point of the regulator 71.

The factor by which the signal arriving in the device 36 through the conduit 85 is multiplied may be manually adjusted and maintained, for example, at a value which is equal to the relation of the total length of one of the tubes 61a to 61] to the portion of the tube 617 between the points 77 and 79. The tubes 61a to 61 usually have the same lengths. The aforesaid factor is equal to the ratio between the mean temperature difference of the fluid between the inlet and the outlet of the heat exchanger and the mean temperature difference between the locations 76 and 79, provided that there is a substantially steady heat supply and heat transfer. Alternatively, a signal may be automatically produced corresponding, for example, to the rate of flow of the fluid passing through the heat exchanger and conducted into the device 86 through a conduit 93.

The apparatus shown in FIG. 4 operates as follows:

If the signal produced in the comparison device 82 and corresponding to the temperature difference between the locations 77 and 76 increases due to an increase of the heating medium indicated by the arrows 78, the temperature set point of the regulator 71 is so adjusted that the injection valve 73 is opened and the temperature at the location 68 is reduced. This may still leave a deviation surface of the heat exchanger.

signal in the conduit 87 for supplemental adjustment of the, set point of the regulator 71.

The heat exchanger forming part of the example .illustrated in FIG. corresponds substantially to that shown in FIG. 4. A plurality of tubes 101a to 101k is arranged in parallel relation with respect to the flow of a heat exchanging medium therethrough. The inlets of the tubes 101a to 191k are connected to an inlet header 1&2 and the outlets are connected to a collector 1ti3. The fiuid enters the header 192 through an inlet conduit 104 and leaves the collector 193 through an outlet pipe 105. For controlling the inlet temperature of the fluid, a quickacting control circuit is provided which includes a thermocouple 196 connected to the inlet conduit 1114, a device 1117 producing a signal corresponding to the temperature sensed by the thermocouple 106 which signal is conducted through a conduit 1118 to .a regulator 1119 producing a signal conducted through a conduit 110 to the motor operator of a coolant injection valve 111 in a pipe 112 for injecting coolant into the inlet conduit 1194. The regulater 1119 has a proportional-integral character. The tem-.

perature of the fluid passing through the tubes 101a to 1M1: is measured by thermocouples 114a to 114k located at the points where the tubes traverse a plane 113. The outlet portions of the tubes 11310, 101 and 101i beginning at a plane 115 are shielded from the heating medium indicated by arrows 116. A device 117 connected to the thermocouples 11 M114) and 114i produces a signal corresponding to the total or the mean value of the temperatures measured by the thermocouples 1140, 114 and144i. The produced signal may be amplified, if desired. A device 118 produces a signal corresponding to the sum or the mean value of the temperatures measured by the thermocouples 114a, 11411, 114a, 1142, 114g, 11% and I 114k. The signals produced by the devices 117 and 118 are conducted to a comparison device 121 through conduits 119 and 12%, respectively. The comparison device 121 is for thepurpose of subtracting the signal produced by the device 117 from the signal produced by the device 118 and of producing a signal corresponding to the result of this subtraction, which signal is conducted through a conduit 122 to a multiplication device 123 which multiplies the signal arriving throughthe conduit 122 by a factor introduced into the device 123 through a conduit 133. The signals produced in the devices 117 and 118 are also conducted through conduits 128 and 127, respectively, to a totalizing device 124 which produces a signal conducted through a conduit 125 to a proportionalintegral regulator 126. The signal produced by the regulator 126 is superimposed at 129 on the signal produced by the multiplication device 123 and the resulting signal is conducted through a conduit 139 into the regulator 109 for adjusting the set point thereof as in the system shown in FIG..4. The regulator 126 may receive a signal through a conduit 134 for modifying the transmission behavior of the regulator. A differential element 132 may be interposed in a conduit 131 by-passing the multiplication device 123. A suitable device for producing theeifect of the element 132 is shown in FIG. 6 and comprises elements 191, 192, 222 to 226, 226a and 22611 which will be described later. This arrangement efiects an amplification of the signal in the conduit according to the speed of change of the mean temperature difference which is measured in the device 121, the effected amplification being limited as to time.

FIG. 6 diagrammatically illustrates a system according to the invention whereby the outlet temperatureof the fluid passing through a heat exchanger is influenced according to a plurality of temperature differences which are measured at different parts of the heat exchange The latter comprises a plurality of tubes 141a to 141 which are arranged in parallel relationwith respect to the flow of a fiuid to be heated-therethrough and to which the fluid is conducted through an inlet conduit142 and a header 143. The fluid heated in the heat exchanger, which may be the superheater 'of a steam generator, is collected ina collector 144 and flows through a pipe 145, 'for example into a turbine, not shown. A portion of the tubes141a, 1 11c and 141.? is shielded against a heating medium indicated by arrows 146, for example by heat insulation 14-7. The tube 141a is insulated at its outlet portion, the tube 141C is insulated at its iniet portion and thetube 141a is insulated at an intermediate portion of the tube.

Electric temperature sensing devices 148, 149 and 15s are provided at the ends of the insulated portions of the tubes 141e, 141a and 141a, respectively. The devices 148, 149 and 156 are arranged in series relation in a con- .duit 151. The electric resistance of the temperature sensitive devices changes in response to the sensed temperatures which correspond to the temperatures of the fluid entering the insulated tube portions, these temperatures being transformed according to the interior heat transfer behavior of the insulated tube portions, the transformation being efiectcd, for example at a change of the temperature of the-fluid, by the heat accumulated inthe tube walls.

Temperature sensing devices 152, 153' and 154 are arranged in series relation in a conduit 155 and connectedto the tubes 141d and 1411) so thata temperature sensing device 148, 14? or 151] at the end of an insulated tube portion is at the same distance from the inlet header v143 as a temperature sensing device 152, 153 or 154 is on a tube having no insulated portion.

The temperature sensing devices 148, 149 and 154i and the temperature sensing devices152, 153 and 154 are made to form part of a Wheatstone bridge by connecting two fixed ,resistors 157 and 158 into the conduits 151 and 155 and applyingelectric alternatingvoltage E to the conduits 151 and 155. The potential between the junction point'159 between the fixed resistors 157 and 152i and the junction point 156 of the conduits151 and 155 is a measure .for the difference between the: mean value of thetemperatures sensed by the devices 152, 153 and 154 and the mean value of the temperatures sensed by the devices .1 36, 14-53 and 150. Thisditfcrence corresponds to the mean temperature difference of the fluid passingythrough the heat exchanger at two spaced locations of the passage, the spacing of thelocations being defined'by the length of the insulated tube portions.

Conductors 166 and 161 are connected at the junction points 156 and 159, respectively,.and are connected to an electric comparison device 162. -By suitable choice of.

The device 162 is provided with a set point adjusting apparatus 178 which is known per se. It inculdes two coils-179 and-18hwoundfin opposite directions and connected in series relation and to a sourceof alternating reference voltage E ,The coils 179 and 180 produce two opposed alternating magnetic fields. A voltage is induced in a third coil 181 which is movable as indicated by arrows 182, the voltage depending on the position of the movablecoil181 and changing its phase 180 when the coil 18,1 passesthrough its middle position. The voltageacrossthe coil 181 is zero when the coil'181 is in its middle position. This voltage is negative when the coil 181 is opposite one of the coils 179 and 180 and is positive when the coil 181 isopposite the other of the coils 179 .or 180. The voltage induced in the. coil 181 is conducted through conductors 183 and 184 to the 9 comparison device 162. The set point voltage-produced by the device 178 can be changed by changing the position of the coil 181 by means of an adjustment screw 185.

The alternating voltage E in relation to the voltage between the conductors 183 and 184 is so chosen as to correspond to the diiference between the inlet temperature and the outlet temperature of the fluid which is heated in the heat exchanger which diiierence is to be expected on the basis of the temperature measurements in the individual tubes 141a to 141 The voltage between the conductors 160 and 161 is subtracted in the conventional manner in the comparison device 162 from the voltage between the conductors 183 and 184 and the resulting voltage is amplified in the conventional manner in the device 162. The amplified voltage across the outgoing conductors 191 and 192 corresponds, for example, to the signal in conduit 87 in the arrangement shown in FIG. 4. The voltage is conducted through a movable coil 217 of an inductive sender 218 and induces in stationary coils 219 and 220 a proportional voltage whose phase and value depend on the position of the movable coil 217 relative to the stationary coils 219 and 220 which are wound in opposite directions. The voltage produced in the sender 218 is received in a conductor 200.

The tube lines 141b, 141d and 141 are provided with additional temperature sensing devices 163, 164 and 165, respectively, which are arranged in series relation in a conductor 166. These temperature sensing devices form part of a second Wheatstone bridge which includes a variable resistor 167 in a line 168 and two fixed resistors 169 and 170. A line 172 is connected to the junction point 171 between lines 166 and 168 and a line 174 is connected to the junction point 173 between the resistors 169 and 170. If there is a voltage E between the conductors 166 and 168 by conducting alternating current to these lines through conductors 175 and 176, a voltage will prevail between the lines 172 and 174 which is proportional to the difierence of a set point adjusted by the resistor 167 for the outlet temperature of the fluid and the actual mean outlet temperature. The voltage between the lines 172 and 174 corresponds to the deviation of the outlet temperature from its set point and is amplified in an amplifier 177.

The amplifier 177 produces a signal in the form of the voltage between output lines 193 and 194 which voltage is applied to a device 195 and to a device 196. The device 195 corresponds to the aforedescribed device 178 and produces a voltage between conductors 200 and 201 which is proportional to the voltage produced in the amplifier 177. The value and phase of the voltage produced in the device 195 can be adjusted by changing the position of a coil 197 relative to stationary coils 198 and 199 by means of an adjusting screw 203. The device 195 may be called a proportional element in the control system, its output signal being proportional to its input signal.

The device 196 serves as an integral sender or signal producer. The lines 193 and 194 are connected in the conventional manner to the driver coil of an eddy-current or electromagnetic disc or two-phase induction motor 204 built as a conventional Ferraris motor used for many years in electrical recorders and whose speed of rotation depends on the input voltage. The motor is provided with a pinion 205 rotating at a speed corresponding to the output voltage of the amplifier 177. The pinion 205 engages a rack 206 connected to a movable coil 207 of a further inductive sender across Whose stationary coils 215 and 216 there is a reference voltage E The voltage induced in the coil 207 depends on the position of the coil and, consequently, on the time during which a voltage acts on the driver coil of the motor 204. Therefore, the coil 207 produces the time integral of the voltage across the driver coil and the device 196 forms an integral sender or element of the control system.

The voltage across the conductors 200 and 201 at the location indicated by a dash-dot line 238 in FIG. 6 is the sum of the outlet voltages of the devices and 196 and is conducted into a comparison and amplifying device 208 as a signal for adjusting the set point of the temperature of the fluid entering the heat exchanger. The device 208 is also connected to a device 209 which is connected to an electric resistance temperature sensing device 241 for producing a voltage between conductors 239 and 240 corresponding to the inlet temperature of the fluid into the heat exchanger, the conductors 239 and 240 connecting the device 209 to the device 208. The latter produces a voltage corresponding to the difference between the voltages across the conductors 239 and 240 and across the conductors 200 and 201. The outlet voltage of the device 208 is conducted as driver voltage to a Ferraris motor 210 which actuates a valve 213 by means of a pinion 211 and a rack 212, according to the amplitude and direction of the driver voltage. The valve 213 controls the flow of a coolant through a pipe 214 into the inlet conduit 142 of the heat exchanger.

The aforedescribed system effects an increase of the coolant supplied through pipe 2114 upon an increase of the outlet voltage of the comparison device 162 which voltage represents the difference between the set point of the outlet temperature and the difference between the inlet and outlet temperatures of the fluid passing through the heat exchanger which ditierence is extrapolated from the measured temperature differences. The remaining deviation of the temperature of the fluid leaving the heat exchanger from the desired temperature is taken care of by the proportional element 195 and the integral element 196. A temperature disturbance in the inlet conduit 142 is taken care of directly by the device 209 which increases the amount of injected coolant upon a sudden increase 'of the temperature in the conduit 142.

Preferably at least one additional control signal is superimposed on the signal acting on the motor 210 which signal corresponds to the speed of the voltage change across the conductors 191 and 192. The signal is produced in a device 221 whose structure corresponds to that of the device 196. However, in the device 221 the driver coil of the Ferraris motor and the movable coil 222 are arranged in series relation between the conductors 191 and 192. The movable coil 222 cooperates with stationary coils 223 and 224 which are wound in opposite directions and across which there is a voltage drop E A change of the voltage between the conductors 191 and 192 produces a voltage between the terminals 226a and 22611 of the driver coil of the motor 226 which voltage initially corresponds to the change of the voltage across the conductors 191 and 192 and thereupon gradually vanishes to zero upon increase of the delayed induced voltage across the coil 222 which is moved by the motor 226 to which it is connected by a rack 225. The voltage across the terminals 226a and 226b and the output voltage of an amplifier 229 between conductors 230 and 231 is approximately proportional to the first derivative or rate action of the control voltage between the conductors 192 and 191. The term first derivative or rate action is described in Automatic Control Terminology prepared by the terminology committee of the Instruments and Regulators Division of the ASME, page 12, items 503, 503a, copyright 1954. The conductors 230 and 231 are connected to a movable coil 232 of an inductive sender 233 whose stationary coils 234 and 235 are interposed in the conductor 200. The position of the coil 232 can be changed by manipulating a hand wheel 235 for adjusting the phase position and the transmission factor of the induced voltage relative to the input voltage. The output voltage of the sender 233 is added to the voltage in the conductor 200.

In a diiferential quotient sender 236 corresponding to the sender 221 a second temporal derivation is formed from the output voltage of the amplifier 229. The output I l voltage of an inductive sender 237 corresponds to the second temporal derivation of the output signal of the comparison device 162.

The devices 221 to 237produce an additional voltage peak between the conductors 206 and 201 immediately after change of the voltage across the conductors 191 and 192, the voltage peak gradually decreasing to zero. This causes a temporary amplification of the signal produced by the device 162 and acting on the injection valve 213 so that the temperature of the fluid entering the heat exchanger is quickly corrected and the inertia of the entire control system is reduced. It is obvious that additional derivations can be superimposed on the control system.

The embodiment of the invention illustrated in FIG. 7 fundamentally corresponds to the embodiment shown in FIG. 2 and like parts are designated by like numerals in FIG. 2 and FIG]. The system shown in FIG. 7 includes the following features over the system shown in FIG. 2:

The outlet signal of the multiplying device 47 is not only conducted through the regulator 39 to the injection valve 35, but is also conducted through a conduit 251 into a device 252. In this device two or more temporal derivations are formed from the signal as is done by the devices 221 to 237 of the system shown in FIG. 6. The produced differential quotients are added in the device 252 and a signal corresponding to the resulting sum is conducted directly through a signal conduit 253 to the location 254 where the signal is added to the signal arriving through the conduit 40. In this way, the signal derivedfrom the difference of the temperatures of the fluid between'the locations 37 and 44 quickly and effectively controls the operation of the valve 35.

Signals corresponding to the actual inlet and outlet temperatures of the fluid passing through the heat exchanger are conducted through conduits 256 and 257 to a comparison device 258 wherein the signal arriving through the conduit 25s is subtracted from the signal arriving through the conduit 257. The signal corresponding to this subtraction is conducted through a conduit 259 into a further comparison device 250 in which the signal produced in the, multiplication device 47 and arriving through a conduit 261'is subtracted from the signal pro- 7 duced in the device 258. The signal produced by the multiplication device 47 corresponds to the difference of the temperatures of the fluid entering and leaving the heat exchanger, i.e. of the temperatures in the conduits 32 and 33, which difference is extrapolated from the actual difference between the temperatures of the fluid at the locations 37 and 44. This extrapolated temperature difference is compared in the device 250 with the actual temperature difference between the locations 37 and 52 and a-signal representing the result of this comparison is conducted through a conduit 262 to an integration device 263. The latter forms the time integral of the signal in the conduit 262, for example, by means of a device as designated by numeral 1% in FIG. 6. The signal produced in the device 263 is conducted through a conduit 264 to the multiplication device 47 for providing the factor by which the signal arriving through the conduit46 is multiplied in the device 47.

As in the system shown in FIG. 2, a signal corresponding to the setpoint of the outlet temperature of the fluid is introduced through conduit 51 and is deducted in a device 265 from the signalarriving through conduit 54 and the so produced signal which corresponds to the devil: factor of the device 47 is continuously adjusted by a signal which acts relatively slowly because of the integral element 263 so that the outlet signal of the multiplication device .47, which extrapolates the difference between the inlet and outlet temperatures of the'fluid from the temperature difference at the locations 37 and 44, corresponds as accurately as possible to the actual difference between the inlet and outlet temperatures of the fluid.

A signal depending on the operating character of the heat exchanger, for example depending on the rate of flow of the fluid through the heat exchanger, may be introduced through conduits 274 to 278 into theelenients 57, 252, 263, 3? and 267, respectively, for affecting the transmission characteristics of these elements. 'Similar to the system shown in FIG. 5 the differential signal in the conduit 253 is directly introduced into the signal con= duit 56 by means of a conduit 279. g

The structure ofthe devices diagrammatically shown in F165. 1, 2, 4, 5 and 7 may correspond to the structures of devices answering the same. purpose shown in FIG. 6. v

The invention is not limited to an implementation by electro-inductive apparatus. The fundamental features of the control system can equally well be produced by equivalent conventional hydraulic or pneumatic apparatus. The invention is not limited to the described and illustrated examples. Itcan be. used in connection with many types of heat exchangers and is not limited to superheaters of steam generators. The temperature of the fluid leaving the heat exchanger may not be influenced by injecting a coolant into the inlet conduit of the heat exchanger, but may be influenced by providing an indirect or surface cooler or a by-pass conduit connected to the inlet conduit whereby the coolant for the cooler or the rate of flow of fluid through the by-pass conduit is controlled similarly to the control of the coolant injecting valve shown by way of example in the illustrated embodiments. The invention is applicable to heat exchangers in which the fluid is not heated but cooled. The fluid passing through the heat exchanger may change its state, for example, may be converted from the liquid state into the vapor state; in this case it is preferredto arrange both temperature sensitive devices for measuring the difference of the tempertaure of the fluid between two spaced points of its passage through the heat exchanger Within a portion ofthe heat exchanger wherein the fluidhas the I same state. The invention is notlimited to temperature 7 sensitive devices of a particular design.

These devices may measure the temperature of the pipe through which the fluid flows or the temperature of the fluid itself.

I claim:

1. A method of controlling the temperature or" a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid'and said first'fluid, the method comprising the steps ofproducing afirst control signal corresponding to the difference between the temperatures of the first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, of producing a second control signal corresponding to the deviation of the temperature of the first fluid leaving the heat exchanger from a predetermined value, of adding said signals for producing a third signal, and of controlling the temperature of the first fiuid as it enters the heat exchanger in response to said third signal.

2. A method of con-trolling the temperature of a first fluid leaving a heat'exchanger wherein heat is exchanged between a second fluid and said first fluid, themethod comprising the steps of producing a first signal corresponding to the diflerence between the temperatures of the first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, 'of producing a second signal corresponding tothe deviation of the temperature of the first fluid leaving the heat exchanger from a predetermined value, of multiplying the first signal by the second signal for obtaining a third signal, and of 13 Controlling the temperature of the first fluid as it enters the heat exchanger in response to said third signal.

3. A method of controlling the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of producing a first signal corresponding to the temperature of the first fluid at a first point of the passage of the first fluid through the heat exchanger, of producing a second signal corresponding to the temperature of the first fluid at a second point of the passage of the first fluid through the heat exchanger downstream of the first point, of delaying said first signal, the delay substantially corresponding to the time required by said first fluid to flow from said first point to said second point, deducting the delayed signal from the second signal for producing a third signal, and of controlling the temperature of the first fluid as its leaves the heat exchanger in response to said third signal.

4. A method of influencing the temperature of a first fluid leaving a heat exchanger wherein said first fluid flows in parallel relation through a plurality of conduits and exchanges heat with a second fluid, the method including the steps of measuring the difference of the temperatures of said first fluid at two spaced locations in each of a plurality of said conduits, of producing a control signal corresponding to the average of said temperature diflerences, and of controlling the temperature of the first fluid entering the heat exchanger in dependence on said signal.

5. A method of influencing the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of measuring the difference of the temperatures of said first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, of extrapolating the diflerence between the temperature of the first fluid entering the heat exchanger and the temperature of the first fluid leaving the heat exchanger from the difference of the temperatures of said first fluid at said two spaced locations, of substracting the extrapolated difference from the temperature of the first fluid required at the outlet of the heat exchanger, of forming a control signal corresponding to the result of said subtraction, and of influencing the temperature of the first fluid leaving the heat exchanger in response to said control signal.

6. A method of influencing the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of measuring the difference of the temperatures of said first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, of etxrapolating the difference between the temperature of the first fluid entering the heat exchanger and the temperature of the first fluid leaving the heat exchanger from the difference of the temperatures of said first fluid at said two spaced locations, and of subtracting the extrapolated difference from the desired temperature of the first fluid as it leaves the heat exchanger, the temperature value resulting from said subtraction forming the set point for controlling the temperature of the first fluid entering the heat exchanger.

7. A method of influencing the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of measuring the diflerence of the temperatures of said first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, of multiplying the difference of the temperatures of said first fluid at said two spaced locations for obtaining a value corresponding to the difference between the temperatures of the first fluid entering and leaving the heat exchanger, and of subtracting said value from the desired temperature of the first fluid as it leaves the heat exchanger, the temperature value resulting from said subtraction forming the set point for controlling the temperature of the first fluid entering the heat exchanger.

8. A method according to claim 7 wherein the factor of said multiplication corresponds to the relation between the mean diiierence between the temperatures of the first fluid entering and leaving the heat exchanger and the mean difference between the temperatures of the first fluid at the two spaced locations.

9. A method of influencing the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of meausring the difference of the temperatures of said first fluid at two spaced locations of the passage of the first fluid through the heat exchanger, of extrapolating the difference between the temperature of the first fluid entering the heat exchanger and the temperature of the first fluid leaving the heat exchanger from the difference of the temperatures of said first fluid at said two spaced locations, of measuring the actual difference between the temperatures of the first fluid enter-- ing and leaving the heat exchanger, of multiplying the difference of the temperatures of the first fluid at the two spaced locations by a factor continuously corrected by the deviation of said actual difference from said extrapolated difference, and of subtracting the results of said multiplication from the desired temperature of the first fluid as it leaves the heat exchanger, the temperature value resulting from said subtraction forming the set point for controlling the temperature of the first fluid entering the heat exchanger.

10. A method of controlling the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between a second fluid and said first fluid, the method comprising the steps of producing a first signal corresponding to the temperature of the first fluid at a first point of the passage of the first fluid through the heat exchanger, of producing a second signal corresponding to the temperature of the first fluid at a second point of the passage of the first fluid through the heate exchanger downstream of the first point, of delaying said first signal substantially inversely proportionally to the rate of flow of said first fluid through the heat exchanger, of deducting the delayed signal from the second signal for producing a third signal, and of controlling the temperature of the first fluid leaving the heat exchanger in response to said third signal.

11. A system for controlling the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between said first fluid and a second fluid and which includes conduit means for conducting said first fluid in heat exchange relation with the second fluid through the heat exchanger, said conduit means having an outlet, the system including means for influencing the temperature of the first fluid passing through the heat exchanger, control means for said temperature influencing means, first temperature responsive means connected to said conduit means for producing a first temperature signal, second temperature responsive means connected to said conduit means downstream of said first temperature responsive means for producing a second temperature signal, a comparison device operatively connected to said temperature responsive means for producing a signal corresponding to the difference between the temperatures to which said temperature responsive means are responsive, said comparison device being operatively connected to said control means for actuating said control means in response to the signal produced by said comparison device third temperature responsive means connected to said outlet, and control signal producing means operatively connected to said third temperature responsive means for producing a control signal corresponding to the deviation of the outlet temperature from a predetermined set point, said control signal producing means being operatively connected to said control means for additionally actuating said control means in response to the signal produced by said control signal producing means.

12. A system as defined in claim 11 wherein at least one of said first two temperatures responsive means is connected to an intermediate part of said conduit means.

13. A system as defined in claim 11 including an inlet pipe connected to said conduit means said means for influencing the temperature of the first fluid including means connected to said inlet pipe for affecting the temperature of said first fluid prior to entering the heat exchanger.

14. A- systeni according to claim 13 wherein said control means including a set point adjusting means connected to said comparison device for adjusting the set point of said control ine'ans according to the signal produced by said cornparison device.

15. A system according to claim 14- including a multiplication device operatively connected to said comparison device for multiplying the signal produced by the latter, a source of a signal corresponding to the set point temperature of the fluid leaving the heat exchanger, a second comparision device operatively connected to said multiplication device and to said source for deducting the multiplied signal from the signal corresponding to the set point temperature of the fluid leaving the heat exchanger to produce a control signal corresponding to the result of said deduction, said second comparison device being opera-tively connected to said set point adjusting means for adjusting the set point of said control means according tosaid control signal.

16. A system according to claim 15 wherein said control signal producing means is operatively connected to said multiplication device for supplying the signal produced by said control signal producing means as multiplication factor to said multiplication device.

17. A system as defined in claim 11 including a delaying means interposed between said first temperature responsive means and said comparison device for delaying transmission of said first temperature signal to said comparison device relative to the transmission of said second temperature signal to said comparison device.

18. A system as defined in claim 11 including means for producing a differential signal corresponding to the speed of change of the signal produced by said comparison device, and means for superposing said differential signal on the signal produced by the comparison device.

19. A system for controlling the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between said first fluid and a second fluid and which includes an inlet, and a plurality of conduits connected to said inlet for conducting a plurality of streams of said first fluid in heat exchange relation with the second fluid through the heat exchanger, said system including means for influencing the temperature of the first fluid passing through the heat exchanger, control means for said temperature influencing means first temperature responsive means connected to one of said conduits for producing a first temperature signal, second temperature responsive means connected to another of said conduits at a point farther away from said inlet than the point of connection of said first temperature responsive means, a comparison device operatively connected to said temperature responsive means for producing a signal corresponding to the difference between the temperatures to which said temperature responsive means are responsive, said comparison device being operatively connnected to said control means for actuating said control means in response to the signal produced by said comparison device. V

20. A system as defined in claim 19 comprising means connected to and preventing heat exchange with a portion of the conduit to which said first temperature responsive means is connected, said first temperature responsive means being connected to the far end of said conduit portion with respect to said inlet for delaying production of said first temperature signal substantially by the time required by the first fluid to flow from the beginning to the end of said tube portion.

21. A system for controlling the temperature of a first fluid leaving a heat exchanger wherein heat is exchanged between said first fluid and a second fluid and which includes an inlet, and a plurality of conduits connected to said inlet for conducting a plurality of streams or said first fluid in heat exchange relation with the second fluid through the heat exchanger, said system including means for influencing the temperature of the first fluid passing through the heat'exchanger, control means for said temperature influencing means, two spaced temperature responsive devices connected to each of a plurality of said conduits for individually measuring the difierences of the temperature of the first fluid between the locations in the individual conduits where said' temperature responsive devices are located, and means operatively connected to said temperature responsive devices for producing a mean value of said differences and connected to saidcontrol means for actuating said control means according to said mean value.

References Cited in the file of this patent UNITED: STATES PATENTS Switzerland Apr. 30, 1957 OTHER REFERENCES Amber etgal; Special Purpose Computers in the Control of Continuous Processes, in"Automatic Control," May 1958, pages 43-48. a 

1. A METHOD OF CONTROLLING THE TEMPERATURE OF A FIRST FLUID LEAVING A HEAT EXCHANGER WHEREIN HEAT IS EXCHANGED BETWEEN A SECOND FLUID AND SAID FIRST FLUID, THE METHOD COMPRISING THE STEPS OF PRODUCING A FIRST CONTROL SIGNAL CORRESPONDING TO THE DIFFERENCE BETWEEN THE TEMPERATURES OF THE FIRST FLUID AT TWO SPACED LOCATIONS OF THE PASSAGE OF THE FIRST FLUID THROUGH THE HEAT EXCHANGER, OF PRODUCING A SECOND CONTROL SIGNAL CORRESPONDING TO THE DEVIATION OF THE TEMPERATURE OF THE FIRST FLUID LEAVING THE HEAT EX- 